Counter-rotating fan

ABSTRACT

A counter-rotating fan includes a first fan including a first impeller including first blades radially arranged around a predetermined center axis, a first motor that rotates the first impeller around the center axis, and a first case surrounding an outer circumference of the first impeller, and a second fan including a second impeller including second blades radially arranged around the center axis, a second motor that rotates the second impeller around the center axis, and a second case surrounding an outer circumference of the second impeller. The first blades include a front edge located foremost in a rotation direction and a rear edge located rearmost in the rotation direction. A radially outermost end of the rear edge is located closer to the second fan than a radially innermost end.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to Japanese Patent Application No. 2018-069445 filed on Mar. 30, 2018. The entire contents of this application are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present disclosure relates to a counter-rotating fan.

2. Description of the Related Art

Conventionally, in electronic devices such as personal computers and servers, a cooling fan is used to cool electronic components in a housing. A serial axial fan in which two air blowing units are connected in series along a predetermined center axis is known as one of such cooling fans. This serial axial fan is formed by arraying two impellers rotating in directions opposite to each other in a direction of the center axis.

A cooling fan having a larger air volume is required in a relatively large electronic device such as a server. For this reason, there is a demand for providing a new technique capable of obtaining the larger air volume even in a counter-rotating fan that is one type of the serial axial fan.

SUMMARY OF THE INVENTION

According to an illustrative embodiment of the present disclosure, a counter-rotating fan includes a first fan including a first impeller including a plurality of first blades radially arranged around a predetermined center axis, a first motor that rotates the first impeller around the center axis, and a first case surrounding an outer circumference of the first impeller, and a second fan including a second impeller including a plurality of second blades radially arranged around the center axis, a second motor that rotates the second impeller around the center axis, and a second case surrounding an outer circumference of the second impeller. The plurality of first blades include a front edge located foremost in a rotation direction and a rear edge located rearmost in the rotation direction. A radially outermost end of the rear edge is located closer to the second fan than a radially innermost end.

The illustrative embodiment of the present disclosure provides a counter-rotating fan having larger air volume.

The above and other elements, features, steps, characteristics and advantages of the present disclosure will become more apparent from the following detailed description of the illustrative embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view illustrating a counter-rotating fan according to an illustrative embodiment of the present disclosure.

FIG. 2A is a plan view illustrating a first axial fan when the first axial fan is viewed from an intake side.

FIG. 2B is a plan view illustrating the first axial fan when the first axial fan is viewed from an exhaust side.

FIG. 3A is a plan view illustrating a second axial fan when the second axial fan is viewed from the intake side.

FIG. 3B is a plan view illustrating the second axial fan when the second axial fan is viewed from the exhaust side.

FIG. 4 is a sectional view illustrating a counter-rotating fan of an illustrative embodiment of the present disclosure.

FIG. 5 is an enlarged sectional view illustrating a main portion along an axial direction of the counter-rotating fan.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, illustrative embodiments of the present disclosure will be described with reference to the drawings. The scope of the present disclosure is not limited to the illustrative embodiments described below, but any change can be made within the scope of the technical idea of the present disclosure.

Sometimes a scale, a number, and the like of a structure in the following drawings differ from those of an actual structure for the sake of easier understanding of the members or portions. In each drawing, a Z-axis is illustrated as appropriate. A Z-axis direction in each drawing is a direction parallel to the axial direction of a center axis J in FIG. 1. In the following description, a positive side (+Z-side, one side) in the Z-axis direction is referred to as an “intake side”, and a negative side (−Z-side, the other side) in the Z-axis direction is referred to as an “exhaust side”. The intake side and the exhaust side are directions used simply for the sake of convenience in the description, but do not restrict an actual positional relationship or a direction. Unless otherwise noted, a direction (Z-axis direction) parallel to the center axis J is simply referred to as an “axial direction”, a radial direction centered on the center axis J is simply referred to as a “radial direction”, and a circumferential direction centered on the center axis J, namely, a direction along a circumference of the center axis J is simply referred to as a “circumferential direction”. In the following description, “in planar view” means a state viewed from the axial direction.

For example, a counter-rotating fan 100 according to the embodiment is used as an electric type cooling fan that air-cools an electronic device such as a server.

FIG. 1 is an exploded perspective view illustrating the counter-rotating fan of the embodiment. FIG. 2A is a plan view illustrating a first axial fan when the first axial fan is viewed from the intake side, and FIG. 2B is a plan view of the first axial fan when the first axial fan is viewed from the exhaust side. FIG. 3A is a plan view of a second axial fan when the second axial fan is viewed from the intake side, and FIG. 3B is a plan view of the second axial fan when the second axial fan is viewed from the exhaust side. FIG. 4 is a sectional view illustrating the counter-rotating fan of the embodiment.

As illustrated in FIG. 1, the counter-rotating fan 100 includes a first axial fan (first fan) 1 and a second axial fan (second fan) 2. In the counter-rotating fan 100, a first axial fan 1 is disposed on the intake side that is one side in the axial direction, and a second axial fan 2 is disposed on the exhaust side that is the other side in the axial direction. That is, in the embodiment, the first axial fan 1 and the second axial fan 2 are sequentially disposed from one side in the axial direction toward the other side in the axial direction.

In the counter-rotating fan 100, a first impeller 10 of the first axial fan 1 and a second impeller 20 of the second axial fan 2 rotate in directions opposite to each other, whereby air is taken from the left side in FIG. 1 (that is, the side of the first axial fan 1) and is delivered to the right side (that is, the side of the second axial fan 2) to generate air flow in a direction of a center axis J.

In the counter-rotating fan 100, by setting a rotation direction of the first impeller 10 and a rotation direction of the second impeller 20 in opposite directions, a high static pressure and a large air volume can be achieved as compared with the serial axial fan in which the two impellers rotate in the same direction.

As illustrated in FIGS. 2A, 2B and 4, the first axial fan 1 includes the first impeller 10, a first motor 11, a first case 12, a plurality of first support ribs (first support members) 13. As illustrated in FIGS. 3A, 3B, and 4, the second axial fan 2 includes the second impeller 20, a second motor 21, a second case 22, a plurality of second support ribs (second support members) 23.

The first impeller 10 includes a plurality (in the embodiment, five) of first blades 10 a radially arranged at equal pitches around the center axis J. The first impeller 10 rotates in a predetermined direction (the direction of an arrow illustrated in FIGS. 2A and 2B) around the center axis J by the first motor 11. The number of the first blades 10 a in the first impeller 10 is not limited to five. The plurality of first blades 10 a include a front edge 10 a 1 located foremost in the rotation direction and a rear edge 10 a 2 located rearmost in the rotation direction. A radially outermost end 10 a 3 of the rear edge 10 a 2 is located closer to the second axial fan 2 than a radially innermost end 10 a 4 is. The rear edge 10 a 2 is brought close to the second axial fan 2 from the radially innermost end 10 a 4 toward the radially outermost end 10 a 3. An axial gap between the rear edge 10 a 2 and the end of the first support rib 13 on the side of the first impeller 10 gradually decreases toward a radially outer region. The first impeller 10 includes a first blade support 10 b that has a covered cylindrical shape and supports the plurality of first blades 10 a, and the radially outermost end of the rear edge 10 a 2 is located closer to the second axial fan 2 than the lower end of the first blade support 10 b is. The radially outermost end of the rear edge 10 a 2 is located closer to the second axial fan 2 than the upper end of a first circuit board 33 is. Because a circumferential speed of the first blade 10 a is faster on the radially outer region, the radially outer region is larger than a radially inner region in an amount of work. With enlarging area of the first blade 10 a, the amount of work of the first blade 10 a increases, and a wind loss also increases, so that efficiency of the first motor 11 is degraded. That is, when the radially outer region of the first blade 10 a performs the work while the radially inner region does not perform the work, both the air volume and the efficiency are increased in a well-balanced manner. As the rear edge 10 a 2 comes close to the first support member 13, a static pressure of the counter-rotating fan 100 is enhanced, but noise increases. By adopting this configuration, the first blade 10 a can perform the work while a distance between the rear edge 10 a 2 and the first support rib 13 is kept constant, and the counter-rotating fan 100 having the large air volume can be provided. In addition, the air discharged from the first impeller 10 reduces an interference sound with the second impeller 20, so that a noise value can be suppressed.

The second impeller 20 includes a plurality (in the embodiment, three) of second blades 20 a radially arranged at equal pitches around the center axis J. The second impeller 20 rotates around the center axis J by the second motor 21 in a direction opposite to the direction of the first impeller 10 (the direction of an arrow in FIGS. 3A and 3B). Consequently, the second impeller 20 generates a flow of air in the same direction as a flow of air by the first impeller 10 (that is, a flow of air in the direction of the center axis J from the left side to the right side in FIG. 1). The number of second blades 20 a in the second impeller 20 is not limited to three. The plurality of second blades include a front edge located foremost in the rotation direction and a rear edge located rearmost in the rotation direction, and the axial distance between a radially outermost end of the front edge of the plurality of first blades and the radially outermost end of the rear edge is larger than the axial distance between the radially outermost end of the front edge of the plurality of second blades and the radially outermost end of the rear edge. As a result, the amount of work in rotationally driving the first impeller 10 is larger than the amount of work in rotationally driving the second impeller 20. The second impeller 20 has a role of discharging the air delivered from the first impeller 10 to the discharge side. On the other hand, the first impeller 10 needs to send the air to the second impeller 20. That is, because the counter-rotating fan 100 depends on the amount of air taken in by the first impeller 10, the amount of work increases easily as a whole when the amount of work of the first impeller 10 increases. Consequently, the air volume can be increased by adopting this configuration.

The first case 12 surrounds the outer circumference (radial outside) of the first impeller 10. In the embodiment, for example, the first case 12 is formed by aluminum die casting. The first case 12 includes a peripheral wall 12A having a tubular shape and extending in the axial direction, four intake-side flanges 12B provided on the intake side in the axial direction of the peripheral wall 12A and protruding toward the radial outside, and four exhaust-side flanges 12C provided on the exhaust side in the axial direction of the peripheral wall 12A and protruding toward the radial outside.

The first case 12 constitutes a wind tunnel by an inner peripheral surface 12A1 formed of a cylindrical surface in the peripheral wall 12A. As illustrated in FIG. 2A, an intake-side end surface 53 in the axial direction of the peripheral wall 12A is flush with a surface 12B1 of the intake-side flange 12B.

As illustrated in FIG. 2B, an exhaust-side end surface 54 in the axial direction of the peripheral wall 12A is flush with a surface 12C1 of the exhaust-side flange 12C. A first notch 14 is provided at one of the four exhaust-side flanges 12C. The first notch 14 is provided so as to penetrate a part of the peripheral wall 12A.

As illustrated in FIGS. 2A and 2B, an outline shape of the first case 12 is substantially quadrangular as viewed in the axial direction. The four corners in the quadrangular outline shape are formed by the outlines of the intake-side flange 12B and the exhaust-side flange 12C. Through-holes 51, 52 penetrating in the axial direction are provided in the intake-side flange 12B and the exhaust-side flange 12C, respectively. Fastening screws used in fixing with the second axial fan 2 are attached to the through-holes 51, 52.

As illustrated in FIG. 2B, the plurality of first support ribs 13 (in the embodiment, four) extend radially around the center axis J, are connected to the first case 12, and support the side of the second motor 21 of the first motor 11. The first support rib 13 is connected to the exhaust side of the inner peripheral surface 12A1 in the peripheral wall 12A of the first case 12, and supports the radial outside of the first motor 11.

The configuration of the first motor 11 will be described later.

The second case 22 surrounds the outer circumference (radial outside) of the second impeller 20. In the embodiment, the second case 22 includes a peripheral wall 22A having a tubular shape and extending in the axial direction, four intake-side flanges 22B provided on the intake side in the axial direction of the peripheral wall 22A and protruding toward the radial outside, and four exhaust-side flanges 22C provided on the exhaust side in the axial direction of the peripheral wall 22A and protruding toward the radial outside.

The second case 22 constitutes a wind tunnel by an inner peripheral surface 22A1 formed of a cylindrical surface in the peripheral wall 22A. As illustrated in FIG. 3A, an intake-side end surface 63 in the axial direction of the peripheral wall 22A is flush with a surface 22B1 of the intake-side flange 22B. A second notch 15 is provided at one of the four intake-side flanges 22B. The second notch 15 is provided so as to penetrate a part of the peripheral wall 22A. The shape of the second notch 15 is the same as the shape of the first notch 14.

As illustrated in FIG. 3B, an exhaust-side end surface 64 in the axial direction of the peripheral wall 22A is flush with a surface 22C1 of the exhaust-side flange 22C.

As illustrated in FIGS. 3A and 3B, the outline shape of the second case 22 is substantially quadrangular as viewed in the axial direction. The four corners of the quadrangular outline shape are formed by the outlines of the intake-side flange 22B and the exhaust-side flange 22C. Through-holes 61, 62 penetrating in the axial direction are provided in the intake-side flange 22B and the exhaust-side flange 22C, respectively. Fastening screws used in fixing with the first axial fan 1 are attached to the through-holes 61, 62. The first axial fan 1 and the second axial fan 2 are integrally fixed to each other by inserting fastening screws (not illustrated) into the through-holes 51, 52 of the first axial fan 1 and the through-holes 61, 62 of the second axial fan 2, thereby constituting the counter-rotating fan 100 (see FIG. 1).

The plurality of second support ribs 23 (in the embodiment, four) extend radially around the center axis J, are connected to the second case 22, and support the side of the first motor 11 of the second motor 21.

Specifically, the second support rib 23 is located on the intake side of the inner peripheral surface 22A1 in the peripheral wall 22A of the second case 22, and supports the radial outside of the second motor 21. The configuration of the second motor 21 will be described later.

The first case 12 and the second case 22 are disposed such that the first notch 14 of the exhaust-side flange 12C of the first case 12 and the second notch 15 of the intake-side flange 22B of the second case 22 are aligned (see FIG. 1). In the embodiment, the first notch 14 and the second notch 15 have the same shape. For this reason, the first notch 14 and the second notch 15 overlap each other in a planar manner when viewed from the axial direction. As described above, in the counter-rotating fan 100 of the embodiment, assemblability can be improved using the first notch 14 and the second notch 15 in the alignment of the first axial fan 1 (first case 12) and the second axial fan 2 (second case 22).

As illustrated in FIG. 4, in the counter-rotating fan 100 of the embodiment, the first motor 11 and the second motor 21 are an outer rotor type motor. The first motor 11 includes a stator 30, a rotor 31, a bearing 32, and a first circuit board 33. The stator 30 includes a base 30 a having a substantially annular shape and centered on the center axis J, a bearing holder 30 b having a substantially cylindrical shape and protruding toward the intake side of the base 30 a, and a stator core 30 c attached to the radial outside of the bearing holder 30 b, and a coil 30 d mounted on the stator core 30 c. The coil 30 d is provided on the stator core 30 c with an insulator interposed therebetween.

As illustrated in FIG. 2B, the first support rib 13 extends radially from the base 30 a, and is connected to the inner peripheral surface 12A1 of the peripheral wall 12A of the first case 12. In the embodiment, the base 30 a and the bearing holder 30 b are made of aluminum similarly to the first case 12. That is, the base 30 a and the bearing holder 30 b are integrally formed by aluminum die casting together with the plurality of first support ribs 13 and the first case 12. Two bearings 32, which are a part of a bearing mechanism, are provided inside the bearing holder 30 b in the axial direction.

The first circuit board 33 is held by being inserted into the bearing holder 30 b of the stator 30. The first circuit board 33 has a substantially annular plate shape, is electrically connected to a lead wire (not illustrated) drawn from the coil 30 d of the stator 30, and controls the rotation of the stator 30. The first circuit board 33 is disposed on the side of the second axial fan 2 of the stator 30. For example, an integrated circuit and a capacitor (not illustrated) are mounted on the first circuit board 33. A first wiring 34 (see FIGS. 1 and 2B) in which a plurality of lead wires are bundled is drawn from the first circuit board 33.

The first circuit board 33 is connected to an external device (not illustrated), such as a power supply, which is provided outside the counter-rotating fan 100, through the first wiring 34.

As illustrated in FIGS. 1 and 2B, the first wiring 34 drawn from the first circuit board 33 is held by the first notch provided in the exhaust-side flange 12C, and drawn to the outside of the first case 12.

The rotor 31 is rotatably journaled around the center axis J through the bearing 32 with respect to the stator 30. The rotor 31 includes a core 31 a made of metal and having magnetism and a substantially covered cylindrical shape centered on the center axis J, a magnet 31 b that has a substantially cylindrical shape and is opposed to the coil 30 d of the stator 30 while fixed to the inside (that is, the inside surface) of a side wall of the core 31 a, a shaft 31 c protruding in the axial direction from a cover of the core 31 a, and a bush 31 d.

The shaft 31 c is rotatably journaled through the bearing 32 while inserted into the bearing holder 30 b. In the rotor 31, the core 31 a and the shaft 31 c are integrally held with the bush 31 d interposed therebetween. The first impeller 10 is fixed to the rotor 31 with the bush 31 d interposed therebetween. Consequently, the first impeller 10 is rotatable together with the rotor 31.

As illustrated in FIG. 4, the second motor 21 has the same structure as the first motor 11. That is, the second motor 21 includes a stator 40, a rotor 41, a bearing 42, and a second circuit board 43. The stator 40 includes a base 40 a having a substantially annular shape and centered on the center axis J, a bearing holder 40 b having a substantially cylindrical shape and protruding toward the intake side of the base 40 a, and a stator core 40 c attached to the radial outside of the bearing holder 40 b, and a coil 40 d mounted on the stator core 40 c. The coil 40 d is provided on the stator core 40 c with an insulator interposed therebetween.

The rotor 41 is rotatably journaled around the center axis J through the bearing 42 with respect to the stator 40. The rotor 41 includes a core 41 a made of metal and having magnetism and a substantially covered cylindrical shape centered on the center axis J, a magnet 41 b that has a substantially cylindrical shape and is opposed to the coil 40 d of the stator 40 while fixed to the inside (that is, the inside surface) of a side wall of the core 41 a, a shaft 41 c protruding in the axial direction from a cover of the core 41 a, and a bush 41 d.

The shaft 41 c is rotatably journaled through the bearing 42 while inserted into the bearing holder 40 b. In the rotor 41, the core 41 a and the shaft 41 c are integrally held with the bush 41 d interposed therebetween. The second impeller 20 is fixed to the rotor 41 with the bush 41 d interposed therebetween. Consequently, the second impeller 20 is rotatable together with the rotor 41.

The second support rib 23 extends radially from the base 40 a, and is connected to the inner peripheral surface 22A1 of the peripheral wall 22A of the second case 22. In the embodiment, the base 40 a and the bearing holder 40 b are made of aluminum similarly to the second case 22. That is, the base 40 a and the bearing holder 40 b are integrally formed by aluminum die casting together with the plurality of second support ribs 23 and the second case 22. Two bearings 42, which are a part of a bearing mechanism, are provided inside the bearing holder 40 b in the axial direction.

The second circuit board 43 is held by being inserted into the bearing holder 40 b of the stator 40. The second circuit board 43 has a substantially annular plate shape, is electrically connected to a lead wire (not illustrated) drawn from the coil 40 d of the stator 40, and controls the rotation of the stator 40. The second circuit board 43 is disposed on the side of the first axial fan 1 of the stator 40. For example, an integrated circuit and a capacitor (not illustrated) are mounted on the second circuit board 43. A second wiring 44 (see FIGS. 1 and 3A) in which a plurality of lead wires are bundled is drawn from the second circuit board 43. The second circuit board 43 is connected to an external device (not illustrated), such as a power supply, which is provided outside the counter-rotating fan 100, through the second wiring 44.

As illustrated in FIGS. 1 and 3A, the second wiring 44 drawn from the second circuit board 43 is drawn to the outside of the second case 22 through the second notch 15 provided in the intake-side flange 22B.

In the embodiment, the first notch 14 and the second notch 15 overlap each other in a planar manner when viewed from the axial direction. That is, the drawing direction of the first wiring 34 from the first circuit board 33 is the same as the drawing direction of the second wiring 44 from the second circuit board 43. In the counter-rotating fan 100 of the embodiment, by setting the drawing directions of the first wiring 34 and the second wiring 44 to the same direction, the first wiring 34 and the second wiring 44 can easily be routed to an external device.

The rotor 41 is rotatably journaled around the center axis J through the bearing 42 with respect to the stator 40. The rotor 41 includes the core 41 a made of metal and having magnetism and a substantially covered cylindrical shape centered on the center axis J, the magnet 41 b that has a substantially cylindrical shape and is opposed to the coil 40 d of the stator 40 while fixed to the inside (that is, the inside surface) of the side wall of the core 41 a, and the shaft 41 c protruding in the axial direction from the cover of the core 41 a. The shaft 41 c is rotatably journaled through the bearing 42 while inserted into the bearing holder 40 b.

When the counter-rotating fan 100 of the embodiment is viewed as a whole, the first case 12 of the first motor 11 and the second case 22 of the second motor 21 contact with each other in the axial direction. In the embodiment, the exhaust-side end surface of the first case 12 and the intake-side end surface of the second case 22 contact with each other in the axial direction. More specifically, the exhaust-side end surface 54 of the peripheral wall 12A of the first case 12 and the intake-side end surface 63 of the peripheral wall 22A of the second case 22 contact with each other in the axial direction.

The surface 12C1 (see FIG. 2B) of the exhaust-side flange 12C in the first case 12 and the surface 22B1 (see FIG. 3A) of the intake-side flange 22B in the second case 22 contact with each other in the axial direction. Consequently, the first case 12 and the second case 22 are brought into contact with each other without any gap in the axial direction.

At this point, a position where the first case 12 and the second case 22 contact with each other in the axial direction is referred to as a first position P1. In the embodiment, an end surface of the first case 12 and an end surface of the second case 22 contact with each other at the first position P1 in the axial direction.

In the embodiment, an axial length H1 of the first case 12 is longer than an axial length H2 of the second case 22. Thus, an axial distance of the wind tunnel constructed with the first case 12 is longer than an axial distance of the wind tunnel constructed with the second case 22. In the embodiment, the axial length of the first blade 10 a of the first impeller 10 accommodated in the first case 12 can be set longer than the axial length of the second blade 20 a of the second impeller 20 accommodated in the second case 22.

In the first axial fan 1 and the second axial fan 2, the end surface located on the exhaust side in the axial direction of the first motor 11 is opposed to the end surface located on the intake side in the axial direction of the second motor 21.

Specifically, the first motor 11 and the second motor 21 are disposed such that the surface 30 a 1 opposite to the bearing holder 30 b in the base 30 a and the surface 40 a 1 opposite to the bearing holder 40 b in the base 40 a are brought close to each other. That is, a gap is formed between the surface 30 a 1 and the surface 40 a 1.

At this point, a position where the first motor 11 and the second motor 21 are opposed to each other in the axial direction is referred to as a second position P2. In the embodiment, the first motor 11 and the second motor 21 are disposed while facing in opposite directions in the axial direction, and the end surfaces (the surface 30 a 1 and the surface 40 a 1) of the first motor 11 and the second motor 21 are opposed to each other at the second position P2 different from the first position P1. In the embodiment, because the gap is formed between the surface 30 a 1 and the surface 40 a 1, the second position P2 corresponds to the center position in the axial direction of the gap.

Specifically, the second position P2 is located closer to one side in the axial direction (intake side) than the first position P1 is. In the embodiment, because the first motor 11 and the second motor 21 have the same configuration, the first motor 11 and the second motor 21 have the same axial dimension.

The first support rib 13 includes a connection surface 13 a (see FIGS. 2B and 4) connecting the first motor 11 (the base 30 a) and the first case 12 (the inner peripheral surface 12A1 of the peripheral wall 12A). The connection surface 13 a is an inclined surface that is inclined toward one side in the axial direction (intake side) from the inner peripheral surface 12A1 toward the base 30 a. In the counter-rotating fan 100 of the embodiment, the first axial fan 1 is formed while the first motor is recessed toward one side in the axial direction (intake side).

The second support rib 23 includes a connection surface 23 a (see FIGS. 3A and 4) connecting the second motor 21 (base 40 a) and the second case 22 (the inner peripheral surface 22A1 of the peripheral wall 22A). The connection surface 23 a is an inclined surface that is inclined toward one side in the axial direction (intake side) from the inner peripheral surface 22A1 toward the base 40 a. In the counter-rotating fan 100 of the embodiment, the second axial fan 2 is formed such that the second motor 21 protrudes toward the other side in the axial direction (exhaust side).

In the embodiment, an end surface of the first support rib 13 and an end surface of the second support rib 23 contact with each other in the axial direction. Specifically, in the counter-rotating fan 100, the connection surface 13 a of the first support rib 13 and the connection surface 23 a of the second support rib 23 contact with each other in the axial direction. The first support rib 13 and the second support rib 23 contact with each other with no gap, so that the first support rib 13 and the second support rib 23 function as a stationary blade 25 in the counter-rotating fan 100. The stationary blade 25 functions as a vane that suppresses the spread of the air delivered from the first impeller 10 in a direction away from the center axis J.

FIG. 5 is an enlarged sectional view illustrating a main part along an axial direction of the counter-rotating fan. As illustrated in FIG. 5, in the stationary blade 25, the first support rib 13 and the second support rib 23 are equal to each other in a sectional area along the axial direction. In the counter-rotating fan 100 of the embodiment, the first support rib 13 and the second support rib 23 are equal to each other in the sectional area along the axial direction.

In the counter-rotating fan 100 of the embodiment, by equalizing the sectional areas along the axial direction of the first support rib 13 and the second support rib 23 to each other, generation of misrun can be reduced in forming the first case 12 and the second case 22 by die casting. Thus, high-precision components can be provided as the first case 12 and the second case 22.

As described above, in the counter-rotating fan 100 of the embodiment, the case contact position (first position P1) is different from the motor facing position (second position P2) in the axial direction, so that commonalization of the motor component can be achieved while the size of the first impeller 10 disposed on the intake side is enlarged. Thus, the motor is commonalized, and the first impeller 10 can be designed larger than the second impeller 20, so that the counter-rotating fan 100 that obtains a larger air volume can be provided.

The cost reduction can be achieved by the commonalization of the motor component.

While the embodiment of the present disclosure is described above, features, a combination of the features according to the embodiment are only illustrative, addition, elimination, and substitution of the configuration, and other changes can be made without departing from the scope of the present disclosure. The present disclosure is not limited to the embodiment.

For example, in the counter-rotating fan 100 of the embodiment, the first axial fan 1 is disposed on the intake side while the second axial fan 2 is disposed on the exhaust side. However, the disposition of the first axial fan 1 and the second axial fan 2 is not limited to this configuration. That is, the first axial fan 1 may be disposed on the exhaust side while the second axial fan 2 may be disposed on the intake side.

According to this configuration, the motors of the two fans are commonalized and the wind tunnel distance on the exhaust side is longer than the wind tunnel distance on the intake side. Thus, a counter-rotating fan having a desired air volume characteristic can be provided.

Features of the above-described preferred embodiments and the modifications thereof may be combined appropriately as long as no conflict arises.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims. 

What is claimed is:
 1. A counter-rotating fan comprising: a first fan including a first impeller including a plurality of first blades radially arranged around a predetermined center axis, a first motor that rotates the first impeller around the center axis, and a first case surrounding an outer circumference of the first impeller; and a second fan including a second impeller including a plurality of second blades radially arranged around the center axis, a second motor that rotates the second impeller around the center axis, and a second case surrounding an outer circumference of the second impeller; wherein the plurality of first blades include a front edge located foremost in a rotation direction and a rear edge located rearmost in the rotation direction; and a radially outermost end of the rear edge is located closer to the second fan than a radially innermost end.
 2. The counter-rotating fan according to claim 1, wherein when the first motor and the second motor are rotationally driven, air is taken in from a side of the first fan, and delivered to a side of the second fan to generate a flow of air in a direction of the center axis.
 3. The counter-rotating fan according to claim 1, wherein the rear edge is moved closer to the second fan from the radially innermost end toward the radially outermost end.
 4. The counter-rotating fan according to claim 3, wherein the first fan includes a first case surrounding a radial outside of the first impeller and a plurality of first support ribs that extend radially around the center axis, are connected to the first case, and support the first motor, and an axial gap between the rear edge and an end on a side of the first impeller of each of the first support ribs decreases toward a radially outer region.
 5. The counter-rotating fan according to claim 1, wherein the first impeller includes a first blade support with a covered cylindrical shape and supporting the plurality of first blades; and the radially outermost end of the rear edge is located closer to the second fan than a lower end of the first blade support.
 6. The counter-rotating fan according to claim 1, wherein the first motor includes a stator, a rotor, a bearing, and a first circuit board, the first circuit board is disposed on a side of the second fan of the stator, and the radially outermost end of the rear edge is located closer to the second fan than an upper end of the first circuit board.
 7. The counter-rotating fan according to claim 1, wherein the plurality of second blades include a front edge located foremost in the rotation direction and a rear edge located rearmost in the rotation direction; an axial distance between the radially outermost end of the front edge of the plurality of first blades and the radially outermost end of the rear edge is larger than an axial distance between the radially outermost end of the front edge of the plurality of second blades and the radially outermost end of the rear edge. 